1. Field of the Invention
The present invention relates generally to fluidic set-point pressure sensors and more particullarly to a fluidic set-point pressure sensor with feedback and temperature compensating characteriatics.
2. Description of the Prior Art
U.S. Pat. No. 4,523,611 filed May 1983 by Drzedwiecki describes a fluidic apparatus for converting the absolute pressure of a pressurized fluid to a differential pressure indicating the fluid pressure relative to a reference pressure. The pressurized fluid is directed asymmetrically into a laminar proportional amplifier (LPA) along a centerline towards a first of two outlets at a velocity determined by the fluid pressure. The LPA includes first and second control inlets disposed on opposite sides of the directed fluid jet and connected to a common source of control fluid. The first control inlet being disposed on the same side as the first outlet and the second control inlet being disposed on the same side as the second outlet. The first and second control inlets include respective first and second donwstream control edges which are asymmetrically disposed on opposite sides of the jet, with the second control edge being disposed closer than the first control edge to the center line. Consequently, the jet is deflected towards the second outlet in accordance with the jet velocities such that the differential pressure generated by the jet between the first and second outlets is zero when the fluid pressure is equal to the reference pressure. The first and second control inlets may include variable fluidic resistors which can be varied to adjust the reference pressure. This device however, is limited in reference pressures that may be obtained as well as being limited in gain.
An improved device is disclosed by Phillippi et al. in U.S. Pat. No. 4,534,383. The invention is similar to the set-point pressure sensor disclosed by Drzedwiecki, however, the device is dimensioned and disposed such that, as the pressure of the pressurized fluid supplied to the LPA is increased from zero, the differential pressure generated by the jet of pressurized fluid between the first and second outlets of the LPA is zero at two different supply pressures. Phillippi et al. utilizes the higher of these two pressures at which the differential pressure generated by the jet between the first and second outlets is zero resulting in a device which can obtain much higher reference pressures as well as much higher gains. Both of the devices described herein above rely on critical asymmetrical arrangements of the elements, such as control edge spacing, in the LPA as well as very high fluidic ground resistors tied to the control inlets. The set-point pressure set in these devices uses the restricted entrainment process through these resistors tied to the control inlet. Because these resistors have a very high value and very small flow passage in relation to the supply nozzle resistance, R.sub.g approximately equals 100R.sub.s, these devices are very sensitive to temperature changes.